Identifying and understanding the geometry of the Hayward and neighboring faults is crucial for determining the earthquake hazard of the San Francisco Bay Area. Although creeping along most of its surface trace, the Hayward fault can and does produce major earthquakes; the last rupture occurred in 1868. Slip on the Calaveras and Hayward faults may be transferred directly between the two faults through a contiguous stepover, requiring reassessment of potential earthquake scenarios on both faults. Seismicity on the fault shows that the near-vertical Hayward Fault dips to the east towards the southern end of its trace (Waldhauser and Ellsworth, 2002). Characteristic repeating earthquakes through the stepover provide an estimate of subsurface creep through the juncture. Incorporating InSAR and updated GPS campaign data into an elastic dislocation model of the East Bay, we evaluated the validity of the seismically proposed geometry and explored the kinematics of a contiguous structure linking the Hayward and Calaveras faults.

A seismic trend east of the southernmost trace illuminates a possible contiguous structure linking the Hayward Fault with the Calaveras Fault to the east (Manaker et al., 2005). Accurate hypocenter locations in the seismic trend between the Hayward and Calaveras faults begin to reveal the structure of this stepover region. A tomographically derived velocity structure of the eastern Bay Area, combined with hypo-dd relocations of clustered events, made better absolute relocations of microseismicity possible (Hardebeck et al., 2007). We used these relocations to define the geometry of fault planes in our dislocation model. The relative relocations of the hypo-dd method, (Waldhauser and Ellsworth, 2002) sharpen fault structures and clearly show the eastward dip of the Hayward fault (Figure 2.6 ).
Additionally, characteristic repeating earthquakes (Nadeau et al., 1994) exist through the stepover (Figure 2.6 ), identifying a relatively narrow band of seismicity within the background seismicity. We consider these repeaters as a proxy for creep, confirming aseismic slip through the stepover.

Figure 2.6:
The Mission trend between the Hayward and Calaveras faults is well defined in the background seismicity. Characteristic repeating earthquakes are superimposed on seismicity and color coded by slip rate. Rectangles are the surface projections of model faults.

Figure 2.7:
Modeled velocities based on our slip inversion are shown in gray. Observed velocities are black. All velocity vectors are shown in reference to station LUTZ

GPS campaigns, conducted over the last year in the Grant network in Halls Valley and the Calaveras network surrounding the Calaveras reservoir, give updated surface velocities for the region, demonstrating deformation consistent with a dipping Hayward Fault. In our slip inversion, we also include GPS data from the Bay Area's permanent network and PS-InSAR (Permanent scatter-interferometric synthetic aperture) data.
We use an elastic dislocation model (Okada, 1985) of Bay Area faults (Bürgmann et al., 2006), incorporating several shallow rectangular dislocations to model the stepover (constructed to be as contiguous as rectangles will allow)(Figure 2.6).
We then perform a joint inversion to solve for slip on these dislocations, finding high slip rates near Fremont and towards the Calaveras. Slip inversions on the step-over structure were robust between InSAR and GPS. In the final inversion, the InSAR data are weighted at 20%. Modeled surface velocities (in gray) and observed velocities (black) are shown in Figure 2.7.
As the stepover patches dive underground, modeled slip increases towards the Calaveras fault, which is conistent with creep rates on that section on the Calaveras (Manaker et al., 2003) and with the slip estimates from the repeaters (Nadeau, 1994). Indeed, the existence of characteristic repeating earthquakes through the stepover indicates that creep actively contributes to the direct transfer of slip from the central Calaveras fault to the southern Hayward fault.

On October 30, at 8:04 local time, a 5.4 earthquake nucleated just south of the Calaveras Reservoir. The epicenter was located on the central Calaveras fault between the Calaveras network to the north and the Grant network to the south. Aftershocks to the south extended into the Grant network, which had been surveyed only two weeks before. Following the earthquake, we set up GPS recievers in the Grant network. Additionally, several nearby continuous GPS sites observed this earthquake.
Coseismic offsets following the Alum Rock earthquake are consistent with right lateral slip. Offsets in the Grant network indicate compression across the fault, although the measurements have large error. Aftershocks continued for several days, and the signal continued to develop for months following the earthquake. Unfortunately, the postseismic signal is too small relative to noise to quantify postseismic deformation.

The Hayward Fault dips into and merges with the Calaveras Fault at depth, affecting hazard scenarios on both faults. This contiguous step-over appears to directly transfer slip between the two faults. Characteristic repeating earthquakes outline this geometry and confirm subsurface creep through the juncture. GPS velocities and PS-InSAR range change rates verify this geometry and constrain slip rates. Additionally, the 2007 Alum Rock earthquake demonstrated that this region is seismogenic. This implies that a large earthquake on the Hayward or Calaveras fault may transfer slip through the step-over, in previously unanticipated rupture scenario.